The present invention relates generally to an apparatus and method for reducing the non-linear distortion of pulses in a high-power optical communication system, and specifically to an apparatus and method for reducing the distortion produced by the interaction between Self Phase Modulation (SPM) and Group Velocity Dispersion (GVD) in an optical system operating at a high power level.
The availability of optical amplifiers with increasing output power capacity has expanded the possibilities for high-power optical communication systems. Before the availability of high-power optical amplifiers, optical transmission systems typically employed relatively low power optical sources for initiating signals within a fiber optic system, and relied on a series of repeaters or amplifiers to regenerate or boost the optical signal along its path. High-power optical amplifiers, on the other hand, permit a reduction in the number of repeaters or amplifiers required along a fiber optic link.
Optical signals traveling in a fiber optic system at high power levels, however, are subject to distortions not evident at lower power levels. In conventional low power systems, a single-mode optical fiber behaves as a lossy, dispersive, linear medium. An optical pulse at a low power level attenuates as it passes along the fiber, and becomes symmetrically broadened due to first-order Group Velocity Dispersion (GVD) if the fiber is sufficiently long, e.g. over 600 km. At transmission rates that approach 100 Gb/s, second-order GVD causes the data pulse to spread asymmetrically as well. Nonetheless, typical optical communication at low power levels results in an overall linear response along a standard transmission fiber.
For high-bit rate systems that have an input power in excess of, for example, 5 mW, a single-mode optical fiber begins to exhibit non-linear distortion characteristics caused by Self Phase Modulation (SPM). As an optical pulse propagates in a transmission fiber at high power levels, SPM generates new frequency components that develop a positive frequency chirp. Interaction between SPM and GVD produces non-linear distortion for an optical pulse that is governed by several parameters. These include the optical peak power level launched into the fiber, the sign and the amount of the dispersion of the transmission fiber, and the dispersion map of the total link (i.e. how the signal accumulates dispersion along the link).
Various publications, including Agrawal, Nonlinear Fiber Optics, Academic Press, 2nd. ed. (1989), describe theoretically the amount of positive chirp produced by SPM on a Gaussian pulse. The power of such a pulse conforms to the following relationship:                               P          ⁡                      (            t            )                          =                              P            0                    ⁢                      exp            ⁡                          [                              -                                                      (                                          T                                              T                        0                                                              )                                                        2                    ⁢                    m                                                              ]                                                          (        1        )            
where P0 is the pulse peak power and T0 is the pulse half-width at 1/e-intensity point. As is readily known in the art, the value m corresponds to the order of the Gaussian pulse. When m=1, the pulse is Gaussian. A larger value of m represents a super-Gaussian pulse, i.e. a sharper Gaussian pulse having shorter rise and fall times. With very high values of m, such as where m greater than  greater than 1, the pulse approaches the shape of a square pulse. With respect to SPM-induced chirp, Agrawal defines it mathematically as follows:                               δ          ⁢                      xe2x80x83                    ⁢                      ω            ⁡                          (              T              )                                      =                                            2              ⁢              m              ⁢                              xe2x80x83                            ⁢                              z                eff                                                                    T                0                            ⁢                              L                NL                                              ⁢                                    (                              T                                  T                  0                                            )                                                      2                ⁢                m                            -              1                                ⁢                      exp            ⁡                          [                              -                                                      (                                          T                                              T                        0                                                              )                                                        2                    ⁢                    m                                                              ]                                                          (        2        )            
where m changes with the shape of the pulse, the effective fiber length zeff is defined as zeff=[1xe2x88x92exp(xcex1z)]/xcex1, z being the fiber length, the nonlinear length is defined as LNL=1/(xcex3P0) and xcex3 is the fiber nonlinearity coefficient. The maximum spectral broadening of the pulse is given by:                               δ          ⁢                      xe2x80x83                    ⁢                      ω            max                          =                                            2              ⁢              m              ⁢                              xe2x80x83                            ⁢                              Φ                max                                                    T              0                                ⁢                                    (                              1                -                                  1                                      2                    ⁢                    m                                                              )                                      1              -                                                1                  /                  2                                ⁢                m                                              ⁢                      exp            ⁡                          [                              -                                  (                                      1                    -                                          1                                              2                        ⁢                        m                                                                              )                                            ]                                                          (        3        )            
where xcfx86max=xcex3P0zeff. Likewise, GVD causes a chirp on an optical pulse in high-power systems. Agrawal defines the GVD chirp as follows:                               δ          ⁢                      xe2x80x83                    ⁢          ω                =                                            2              ⁢                              sgn                ⁡                                  (                                      β                    2                                    )                                            ⁢                              (                                  z                  /                                      L                    D                                                  )                                                    1              +                                                (                                      z                    /                                          L                      D                                                        )                                2                                              ⁢                      xe2x80x83                    ⁢                      T                          T              0              2                                                          (        4        )            
where LD=T02/|xcex22| is the dispersion length for the pulse and xcex22 the group velocity dispersion parameter.
A. Naka et al., xe2x80x9cIn-line Amplifier Transmission Distance Determined by Self-Phase Modulation and Group-Velocity Dispersion, xe2x80x9d Journal of Lightwave Technology, Vol. 12, No. 2, pp. 280-287 (February 1994) numerically analyze the propagation of intensity-modulated signal in an optical fiber, taking self-phase modulation, group-velocity dispersion, and 2nd-order group-velocity dispersion into account. Transmission distances yelding a prescribed eye-opening penalty are shown to relate to three characteristic lengths: the dispersion length, the 2nd-order dispersion length, and the nonlinear length.
U.S. Pat. No. 5,539,563 (Park) disclose a system and method for simultaneously compensating for chromatic dispersion and self phase modulation in optical fibers. At least one dispersion compensating (DCF) fiber is utilized to compensate for chromatic dispersion of an externally modulated signal carried by at least one single-mode, standard fiber optical cable. The signal power launched in the DCF fiber is controlled such that precise compensation for the SPM effect in the standard fiber can be achieved.
Other references also discuss the impact of SPM and GVD on optical communications with respect to pulse compression devices and techniques. Peter et al., xe2x80x9cCompression of Pulses Spectrally Broadened by Self-Phase Modulation Using a Fiber-Grating: A Theoretical Study of the Compression Efficiency,xe2x80x9d Optics Communications, Vol. 112, pp. 59-66 (Nov. 1, 1994), discusses a theoretical analysis of the potential for using short-fiber gratings with constant grating period for the compression of optical pulses spectrally broadened by SPM. For fiber gratings with constant grating period, this paper confirms through theory and simulations that the maximum achievable pulse compression factor is practically independent of the grating parameters and is typically on the order of two.
Stern et al., xe2x80x9cSelf-Phase Modulation and Dispersion in High Data Rate Fiber-Optic Transmission Systems, xe2x80x9d Journal of Lightwave Technology, Vol. 8, No. 7, pp. 1009-16, (July 1990), describes the limitations caused by the interaction of first and second-order GVD and intensity-dependent SPM. The paper investigates the theoretical transmission limits imposed by these effects for a range of wavelengths around the zero dispersion wavelength xcex0 for fibers in which polarization dispersion is negligible. The paper finds that operating at wavelengths longer than xcex0 improves the transmission distance for data rates greater than 50 Gb/s due to the cancellation of first-order dispersion by SPM. Above 100 Gb/s, higher order dispersion limits the transmission distance even at wavelengths equal to or longer than xcex0. The paper concludes that linear dispersion compensation using a grating-telescope combination can significantly improve system performance for wavelengths where first order dispersion dominates.
These references, however, focus on the performance of relatively smooth Gaussian pulses in optical systems.
Applicant has observed that modulated optical pulses in a link having less than 600 km of optical fiber do not face pulse overlap due to GVD pulse spreading as considered by the literature for very long distances, even at relatively high bit rates of 2.5 Gb/s. Applicant has further identified that the amount of frequency chirping depends heavily on the shape of the pulses, in particular the pulse edges, which in turn depend on the type of transmission equipment used. Moreover, Applicant has discovered that modulated optical pulses from many conventional SDH and SONET-based transmitters are quite different from smooth Gaussian pulses considered by theoretical calculations, but rather have sharp rising and falling edges similar to super-Gaussian pulses. Applicant remarks that pulses with sharp rising and falling edges are normally preferred for optical communications, in order to minimize the effect of phase jitter and improve detection. These pulses, as observed by Applicant, are subject to frequency chirping much more than theoretical Gaussian pulses. Furthermore, Applicant has found that the pulse shape and the degree of sharpness of their rising and falling edges is not the same for different transmitters and depends on the used equipment.
Moreover, Applicant has determined that due to the above frequency chirping the bit error rate (BER) at the receiver for such pulses is influenced by the receiver characteristics, in particular by the type of electric filtering done in the receiver. This makes the optical system characteristics very much dependent on the choice of transmitting and receiving equipment or the degree of matching of the available transmitter and receiver.
U.S. Pat. No. 5,267,073 (Tamburello et al.) discloses adapters for interconnecting fiber lines including optical amplifiers, wherein the transmitters and receivers have different operating parameters (e.g., transmission speed, wavelength and wavelength variation with temperature) from the operating parameters of the optical amplifiers. An adapter group comprises converting means for converting optical signals to electrical signals, a signal laser transmitter, an adjustment module comprising laser piloting means connected to the output of the converting means and adapted to control the signal transmitter by said electrical signals and an optical amplifier coupled to the output of the laser transmitter.
U.S. Pat. No. 5,504,609 (Alexander et al.) discloses an optical remodulator for converting channel wavelengths and a wavelength division multiplexing system. The ""609 patent discloses a remodulator that includes an electro-optical converter that produces an electrical signal from a received optical signal having a wavelength xcexTi. The electrical signal is amplified by transimpedance amplifier, passed through a filter to limit the noise bandwidth and waveshape the signal, and further amplified by a limiting amplifier. Optionally, the remodulator in the ""609 patent can include a clock and data recovery circuit for use with high data rate signals. A switch automatically selects high data rate signals and passes them through the clock and data recovery element. The remodulator further includes a laser for producing a carrier signal xcexj and an external modulator.
The ""609 patent, however, does not address the operation of a communication link at high input power and does not address distortion from the interaction of SPM and GVD. For handling high data rate signals, the ""609 patent in FIG. 2 shows an optional path for high data rate signals and low data rate signals. For high data rate signals, the switch directs the electrical signal to the clock and data recovery circuit. This circuitry does not include any device positioned after the clock and data recovery circuit for smoothing sharp edges on pulses produced by the clock and data recovery circuit. The ""609 patent does not disclose other techniques for handling high data rate signals.
Applicant has noticed that the SPM-GVD nonlinear interaction can improve or degrade the transmitted signal depending on the values of the parameters in these presented theoretical expressions. In order to understand the influence of SPM and GVD, it is important to evaluate LNL, LD and zeff from the equations above. These parameters identify the length scales over which the nonlinear, the dispersive and the attenuation phenomena become important.
Applicant has determined that a distortion arising due to pulses having sharp rising and falling edges becomes undesirably high if the total length LT of the optical link is greater than a length LM given by                               L          M                =                                            L              NL                                      z              eff                                ⁢          z                                    (        5        )            
where z is the (average) length of the span of fiber between consecutive amplifiers, or the span length in case of a single span.
The present invention is directed to a method and apparatus for reducing non-linear distortion in an optical transmission system caused by the interaction of SPM and GVD that substantially obviates one or more of the limitations and disadvantages of the described prior arrangements. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
In general, the present invention involves techniques for causing the shape of optical pulses that modulate an optical carrier in an optical transmission system to be relatively independent from the shape of those same pulses that are received by equipment that performs the modulation. In particular, a transponder consistent with the present invention includes circuitry for rounding the edges of an optical pulse received from a system transmitter so that the pulses used for modulation resemble Gaussian shaped pulses.
To achieve these and other objects and advantages, and in accordance with the purpose of the invention as embodied and broadly described herein, the invention is, in a first aspect, an optical communication system for reducing non-linear distortion caused by interaction between Self-Phase Modulation and Group Velocity Dispersion including a transmitter for sending optical pulses at a first wavelength; a transponder coupled to receive and convert the optical pulses to a second wavelength, including an opto-electrical device, means for smoothing rise and fall transitions of electrical pulses received from the opto-electrical device, an electrical amplifier, an optical source and an electro-optic modulator; a plurality of spans linearly coupled to the transponder, each having a length z of optical transmission fiber and at least one optical amplifier; the total length of said plurality of span being greater than (LNL/zeff)z, wherein LNL is the fiber nonlinear length and zeff is the effective span fiber length; a receiver coupled to the plurality of spans.
According to a second aspect the invention is an optical communication system for reducing non-linear distortion caused by interaction between Self-Phase Modulation and Group Velocity Dispersion including a transmitter for sending optical pulses at a first wavelength; a transponder coupled to receive and convert the optical pulses to a second wavelength, including an opto-electrical device, means for smoothing rise and fall transitions of electrical pulses received from the opto-electrical device, an electrical amplifier, an optical source and an electro-optic modulator; a section of optical transmission fiber having an effective length zeff greater than the fiber nonlinear length LNL; a receiver coupled to the fiber section.
In both the above first and second aspect of the invention the means for smoothing the electrical pulses can comprise an electrical attenuator positioned between the opto-electronic device and the electrical amplifier or a low pass filter positioned after the electrical amplifier. In an embodiment, the means for smoothing the electrical pulses comprises a data and clock recovery circuit positioned between the opto-electronic device and the electrical amplifier and a low pass filter positioned after the electrical amplifier.
In another aspect, the invention is a method for reducing nonlinear optical distortion caused by the interaction of Self-Phase Modulation and Group Velocity Dispersion, comprising the steps of: receiving optical pulses from a transmitter and converting the optical pulses to electrical pulses; amplifying the electrical pulses; smoothing edges of rise and fall transitions of the electrical pulses; modulating an optical carrier signal with the electrical pulses; and transmitting the modulated optical carrier signal across a plurality of transmission spans having a cumulative length longer than LNL/zeff, wherein LNL is the fiber nonlinear length and zeff is the effective spanfiber length. Preferably, the method further includes the step of compensating for dispersion of the modulated optical carrier signal at a position along the plurality of transmission spans, for example using a chirped fiber grating.
In still another aspect the invention is a transponder for receiving optical pulses at a first wavelength generated by an optical transmitter, modulating an optical carrier with the optical pulses, and alleviating non-linear distortion caused by interaction between Self-Phase Modulation and Group Velocity Dispersion, comprising: a photodiode optically coupled to receive and convert the optical pulses to electrical pulses; an electrical amplifier, operating in a saturation condition, electrically coupled to receive and amplify the electrical pulses; a low-pass filter electrically coupled to receive the electrical pulses from the electrical amplifier, the low-pass filter causing rise and fall times of the electrical pulses to lengthen; an optical source providing an optical carrier at a second wavelength; and an electro-optic modulator positioned to modulate the optical carrier with the electrical pulses from the low-pass filter.
In a further aspect the invention is a transponder for receiving optical pulses at a first wavelength generated by an optical transmitter, modulating an optical carrier with the optical pulses, and alleviating non-linear distortion caused by interaction between Self-Phase Modulation and Group Velocity Dispersion, comprising: a photodiode optically coupled to receive and convert the optical pulses to electrical pulses; an electrical attenuator electrically coupled to receive the electrical pulses from the photodiode; an electrical amplifier, operating in a saturation condition, electrically coupled to receive and amplify the electrical pulses from the electrical attenuator; an optical source providing an optical carrier at a second wavelength; and an electro-optic modulator positioned to modulate the optical carrier with the electrical pulses from the electrical amplifier.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.